The field of this invention is sailboats and more particularly sloop-rigged sailboats in which the sails are supported in a novel manner to yield enhanced performance in all regimes of sailing. While the invention has particular utility in connection with sailboats, it is relevant and applicable to other air, land and water craft which exploit the interaction of wind with soft airfoils to produce lift or driving forces, either in static or dynamic conditions. Examples of such other craft would be airfoil-equipped air-dropped materiel including certain classes of smart munitions, compact or foldable lightweight aircraft such as remotely piloted aircraft, parachutes, and windmills.
Most modern high performance sailboats are sloops, the best known examples being the 12 meter yachts used in all recent America's Cup races. Conventional sloop design comprises a single mast located forward of amidships with a support-system comprising guy wires known as stays and shrouds connected between the mast and the deck. One or more horizontal compression members known as spreaders may be attached crosswise to the mast and connected to the shrouds to provide a triangular, truss configuration for better support of the mast.
A sloop's sails, in all sailing regimes except sailing downwind, act as airfoils which generate lift just as do the wings of an aircraft. The wind which flows along the convex or leeward side of the sail must travel farther than the wind which flows on the opposite side thereof and hence the air pressure is reduced on the convex side. This air pressure differential creates lift which tends to pull the sail and the boat generaly at right angles to the plane of the sail. This lift force can be resolved into a forward component and a sideways component. The undesired sideways component is opposed by the hydrodynamic lift forces of the keel, the hull and the rudder. Thus sloops and other sailboats when sailing close to the wind can have a substantial upwind component of velocity. Optimum lift requires an optimum angle of incidence (or angle of attack) of the apparent wind with the luff of the sail. Also, the sail camber, or fatness, must be varied with apparent wind velocity in order to control the power produced by the sail. For example, in high wind conditions, when sailing close to the wind, a sail with a small camber is required to prevent over powering. Thus, sail flattening can be used to depower sails and reduce the heel angle of the boat. Also, the change in apparent wind direction and velocity with elevation above the water requires that the mid and upper portions of sails be twisted forward relative to the lower portions to optimize lift and thus propulsive forces.
The conventional sloop rigging described above has the serious disadvantage that the mast induces air turbulence along the leading edge, or luff, of the mainsail attached thereto. Such turbulence destroys the laminar flow of air over a substantial portion of the convex or leeward side of the sail and thus greatly reduces the air velocity which is essential for the production of lift. The present invention comprises a rigging system for sloops which removes the mast from the luff area of the mainsail to eliminate the turbulence and improve the lift characteristics of the sail.
Prior art efforts to reduce mast-induced turbulence have involved utilizing very thin masts which required extensive bracing and support in the form of highly tensioned stays and shrouds with long spreaders. The high tension in the stays and shrouds can cause hull distortion and in rough seas the highly tensioned wires can cause mast bending or bowing and even mast failure, with disastrous consequences. Also, the long spreaders restrict the close hauling of the headsail or jib which is necessary to achieve optimum performance when sailing close to the wind. Thus, some sloops have been designed with thin masts as well as short spreaders, which must be compensated for with greatly increased stay and shroud tension with its attendant disadvantages noted above.
The present invention alleviates, to a great extent, these opposing design requirements and provides virtually turbulence-free entry for the mainsail, while providing a strong mast with only moderately tensioned structural elements, which utilize a narrow support base. These features, in addition to providing high lift, will reduce hull construction cost and weight.
The prior art includes numerous sail rigs which remove the mast from the luff area of the mainsail. One of these is shown in U.S. Pat. No. 4,044,702, issued on Aug. 30, 1977. That patent shows in one embodiment a tripod mast with the head of the sail attached to the tripod apex and the foot thereof attached to a boom which rotates atop a short, deck-mounted mast which is directly below the tripod apex. The luff and leech edges of the sail are supported by cables extending from the ends of boom to the tripod apex. Another embodiment utilizes a more or less conventional mast with a boom offset therefrom by means of a short spar which pivots around the mast. The tripod embodiment of that patent is not applicable to the present invention since it has no headsail and thus loses the efficiency of a sloop when sailing "on the wind" and similarly it loses the efficiency of the "unblanketed spinnaker" when sailing "off the wind." The second embodiment described above which might be adaptable to sloops with conventional hulls, would not be adaptable to bowing or bending of the mast which is often desirable to flatten the sail to reduce power and heeling moment.
U.S. Pat. No. 4,273,060, issued in June 16, 1981 shows an A-frame mast mounted on an omnidirectional hull, with a sail fixed in position within the A-frame, the boat heading being varied to change the angle of the sail to the wind. This design does not permit a headsail or jib to be used, which is essential if maximum lift and efficiency is to be obtained. This patent thus does not relate to a sloop.